Method and apparatus for dimmable led driver

ABSTRACT

Aspects of the disclosure provide a method for driving dimmable load. The method includes detecting a dimming characteristic in an energy source from which a load draws a first energy according to the dimming characteristic. The dimming characteristic requires a second energy in addition to the first energy to be drawn from the energy source to sustain an operation of the energy source. The method further includes biasing a switch to consume the second energy. The second energy and the first energy are drawn from the energy source to sustain the operation of the energy source.

INCORPORATION BY REFERENCE

This application is a continuation of U.S. application Ser. No.14/247,556, filed Apr. 8, 2014, which claims the benefit of U.S.Provisional Application No. 61/819,239, “CONTROL METHOD AND SILICONIMPLEMENTATION FOR DIMMABLE LED DRIVER” filed on May 3, 2013. Thedisclosures of the applications referenced above are incorporated hereinby reference in their entireties.

BACKGROUND

Light emitting diode (LED) lighting devices provide the advantages oflow power consumption and long service life. Thus, LED lighting devicesmay be used as general lighting equipment to replace, for example,fluorescent lamps, bulbs, halogen lamps, and the like.

SUMMARY

Aspects of the disclosure provide a method for driving dimmable load.The method includes detecting a dimming characteristic in an energysource from which a load draws a first energy according to the dimmingcharacteristic. The dimming characteristic requires a second energy inaddition to the first energy to be drawn from the energy source tosustain an operation of the energy source. The method further includesbiasing a switch to consume the second energy. The second energy and thefirst energy are drawn from the energy source to sustain the operationof the energy source.

According to an aspect of the disclosure, the switch is switched on/offto draw the first energy from the energy source. In an embodiment, theswitch is implemented using a metal-oxide-semiconductor field-effecttransistor (MOSFET). The MOSFET is biased in a saturation mode toconsume the second energy. The method further comprises biasing theMOSFET in a linear mode to turn on the MOSFET to store the first energyin a magnetic component in connection with the MOSFET, and biasing theMOSFET in an off mode to turn off the MOSFET to transfer the firstenergy from the magnetic component to the load.

In an embodiment, the method includes outputting a first voltage for agate terminal of the MOSFET to bias the MOSFET in the linear mode,detecting a current flowing through the MOSFET, outputting a secondvoltage for the gate terminal of the MOSFET to bias the MOSFET in thesaturation mode in order to consume the second energy when the currentreaches a limit and outputting a third voltage for the gate terminal ofthe MOSFET to turn off the MOSFET.

In an example, the switch is a first switch, and the method furtherincludes switching on the first switch and a second switch to store thefirst energy in a magnetic component and switching on the first switchand switching off the second switch to charge a capacitor that storesenergy for driving an integrated circuit.

Aspects of the disclosure provide a circuit including a control circuit.The control circuit is configured to detect a dimming characteristic inan energy source from which a load draws a first energy according to thedimming characteristic. The dimming characteristic requires a secondenergy in addition to the first energy to be drawn from the energysource to sustain an operation of the energy source. Further, thecontrol circuit is configured to bias a switch to consume the secondenergy. The second energy and the first energy are drawn from the energysource to sustain the operation of the energy source.

Aspects of the disclosure provide an apparatus that includes a magneticcomponent, a switch and an integrated circuit (IC) chip. The magneticcomponent is for transferring energy from an energy source to a load.The switch is for controlling the magnetic component. The IC chip has acontrol circuit on the chip. The control circuit is configured to detecta dimming characteristic in the energy source from which the load drawsa first energy according to the dimming characteristic. The dimmingcharacteristic requires a second energy in addition to the first energyto be drawn from the energy source to sustain an operation of the energysource. Further, the control circuit is configured to bias the switch toconsume the second energy. The second energy and the first energy aredrawn from the energy source to sustain the operation of the energysource.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of this disclosure that are proposed as exampleswill be described in detail with reference to the following figures,wherein like numerals reference like elements, and wherein:

FIG. 1 shows a block diagram of an electronic system 100 according to anembodiment of the disclosure;

FIG. 2 shows a flow chart outlining a process example 200 according toan embodiment of the disclosure; and

FIG. 3 shows a plot 300 of waveforms according to an embodiment of thedisclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a block diagram of an electronic system 100 according to anembodiment of the disclosure. The electronic system 100 operates basedon an alternating current (AC) voltage V_(AC) provided by an AC powersupply 101 with or without a dimmer 102. The AC power supply 101 can beany suitable AC power supply, such as 60 Hz 110V AC power supply, 50 Hz220V AC power supply, and the like.

According to an aspect of the disclosure, the electronic system 100 isoperable under various dimming characteristics of the power supply. Inan example, a power supply may have a pre-installed phase-cut dimmer102, such as a triode for alternating current (TRIAC) type dimmer havingan adjustable dimming angle α. The dimming angle a defines a size of aphase-cut range during which the TRIAC is turned off. Further, a phaserange that is out of the phase-cut range can be referred to as aconduction angle during which the TRIAC is turned on. During an ACcycle, when the phase of the AC voltage V_(AC) is in the phase-cutrange, the TRIAC is turned off. Thus, an output voltage of the dimmer102 is about zero. When the phase of the AC voltage V_(AC) is out of thephase-cut range (e.g., in the conduction angle), the TRIAC is turned on.Thus, the output voltage of the dimmer 102 is about the same as the ACvoltage V_(AC). The phase-cut dimmer 102. can be a leading edge TRIAC, atrailing edge dimmer, or other types of dimmer.

Generally, the TRIAC type dimmer 102 requires a holding current, such asin a range of 8 to 40 mA, and the like, to sustain the currentconduction during the conduction angle. In an example, when a currentdraw from the TRIAC type dimmer 102 during the conduction angle is lowerthan the holding current, such as in a deep dimming situation, the TRIACwithin the dimmer 102 may be prematurely turned off which may causeflicking and shimmering by a light device for example, and causeunpleasant user experience.

According to an aspect of the disclosure, the electronic system 100drives a load 109 that is a power efficient device. In an example, theload 109 is a light emitting diode (LED) lighting device, and the powerfor driving the LED lighting device in the deep dimming situation doesnot sustain the holding current during the conduction angle. Accordingto the aspect of the disclosure, the electronic system 100 allows anexisting circuit component, such as an existing switch, and the like, toconsume additional power in the deep dimming situation to sustain theholding current, and thus the TRIAC type dimmer 102 operates properly inthe deep dimming situation without being prematurely turned off forexample.

In the FIG. 1 example, the electronic system 100 includes a rectifier103, a control circuit 110, an energy transfer module 120, and a load109. These elements are coupled together as shown in FIG. 1. Generally,the energy transfer module 120 includes one or more switches, and thecontrol circuit 110 switches on/off the switches to transfer energy fromthe power supply to the load. According to an aspect of the disclosure,when the power supply is in a deep dimming situation, the controlcircuit 110 biases at least one of the switches to consume additionalpower on the switch in order to sustain the holding current for theTRIAC type dimmer 102, and thus the TRIAC type dimmer 102 operatesproperly in the deep dimming situation without being prematurely turnedoff for example

Specifically, in the Fig, 1 example, the rectifier 103 rectifies an ACvoltage to a fixed polarity, such as to be positive. In an example, therectifier 103 is a bridge rectifier. The bridge rectifier 103 receivesthe output voltage of the dimmer 102, and rectifies the received voltageto a fixed polarity, such as to be positive. The electronic system 100may include a capacitor filter (not shown) to remove high frequencynoise in the rectified voltage V_(RECT). The rectified voltage V_(RECT)is provided to the following circuits, such as the control circuit 110,the energy transfer module 120, and the like, in the electronic system100.

The energy transfer module 120 transfers electric energy provided by therectified voltage V_(RECT) to one or more load devices, such as the load109 and the like. In an embodiment, the energy transfer module 120 isconfigured to use a magnetic component, such as a transformer, aninductor, and the like to transfer the electric energy. The energytransfer module 120 can have any suitable topology, such as a fly-backtopology, a buck-boost topology, and the like. In the FIG. 1 example,the energy transfer module 120 includes an inductor L, a first switchQ1, a second switch Q2, a current sensing resistor R4, a diode D2 and acapacitor C2. These components are coupled to the power supply (e.g.,the rectified voltage V_(RECT)) and the load 109 in a buck-boosttopology as shown in FIG. 1 to drive the load 109. It is noted that theenergy transfer module 120 can be modified to use other suitabletopology to transfer the electric energy.

Generally, in the FIG. 1 example, when the first switch Q1 and thesecond switch Q2 are switched on (e.g., conductive), the inductor L, thefirst switch Q1, the second switch Q2 and the current sensing resistorR4 form a current path from the power supply to the ground, the powersupply charges the inductor L, and the inductor stores electric energy.When the first switch Q1 and the second switch Q2 are switched off(e.g., non-conductive), the electric energy stored in the inductor L isdischarged to the load 109 and the capacitor C2. The capacitor C2.stores the electric energy. The electric energy stored in the capacitorC2 can be provided to the load 109 during the time duration when thefirst switch Q1 and the second switch Q2 are switched on. When the firstswitch Q1 and the second switch Q2 are switched on/off fast, theinductor L is charged and discharged slightly in each cycle, and arelatively steady voltage to the load 109 can be maintained.

It is noted that the first switch Q1 and the second switch Q2 can berespectively switched on/off for other purpose. In the FIG. 1 example, aportion of the electronic system 100, such as the control circuit 110,the second switch Q2, the current sensing resistor R4, and the like isintegrated on an integrated circuit (IC) chip 106. Generally, the ICchip 106 operates under a DC voltage supply VDD. In the electronicsystem 100, a diode D3 and a capacitor C3 are coupled with the energytransfer module 120 as shown in FIG. 1 to form a voltage supply circuit.In an example, when the second switch Q2 is switched off and the firstswitch Q1 is switched on, the diode D3 can be forward biased, and thecapacitor C3 is charged via the inductor L, the first switch Q1, and theforward biased diode D3 to store electric energy. The stored electricenergy on the capacitor C3 can be provided to the IC chip 106 in theform of the DC voltage supply VDD.

It is noted that when the second switch Q2 is switched on, the diode D3is reverse biased to avoid discharging energy stored on the capacitorC3. In an example, when the first switch Q1 and the second switch Q2 aresuitably controlled, the DC supply voltage VDD can be maintainedrelatively stable.

According to an embodiment of the disclosure, the first switch Q1 isable to be controlled to consume relatively large power. In the FIG. 1example, the first switch Q1 is a metal-oxide-semiconductor field-effecttransistor (MOSFET), such as an N-type MOSFET. In an example, when anMOSFET is biased in a saturation mode, the MOSFET has a relatively largevoltage drop over the drain terminal and the source terminal (e.g., V5),and the MOSFET flows a relatively large current from the drain terminalto the source terminal (e.g., I_(Q)). Thus, in the saturation mode, theMOSFET itself consumes a relatively large power (e.g., V5×I_(Q)). In anexample, the MOSFET converts the electric energy into thermal energy.

According to an aspect of the disclosure, the first switch Q1 can bebiased into three operation modes—an off mode, a linear mode and asaturation mode. For example, when the gate voltage of the N-type MOSFETis low, such as about the ground level, the N-type MOSFET is turned offand does not conduct current, thus the N-type MOSFET is biased in theoff mode. When the gate voltage of the N-type MOSFET is relatively high,such as about the same level as the rectified voltage V_(RECT), the gatevoltage is larger than the drain voltage and the source voltage of theN-type MOSFET by at least a threshold voltage of the N-type MOSFET dueto a voltage drop on the inductor L, the N-type MOSFET is turned on witha relatively small source-drain voltage, and the N-type MOSFET is biasedin the linear mode. When the gate voltage of the N-type MOSFET isbetween the drain voltage and the source voltage of the N-type MOSFET,the N-type MOSFET is turned on with a relatively large source-drainvoltage, and the N-type MOSFET is biased in the saturation mode.

The control circuit 110 detects various parameters in the electronicsystem 100 and dynamically adjusts control signals based on the detectedparameters to control the operations of the first switch Q1 and thesecond switch Q2, and thus the control circuit 110 controls theoperations of energy transfer module 120 to transfer the electric energyto the load 109.

In an example, when the dimming angle is zero, for example when thedimmer 102 does not exist, the control circuit 110 uses a constantturn-on time algorithm to generate pulse width modulation (PWM) signalsto control the operations of the first switch Q1 and the second switchQ2. In another example, when the dimming angle is not zero but smallerthan a threshold, the dimmer 102 exists but not in the deep dimmingsituation, the control circuit 110 uses a constant peak currentalgorithm to generate PWM signals to control the operations of the firstswitch Q1 and the second switch Q2. In another example, when the dimmingangle is larger than the threshold, the dimmer 102 is in the deepdimming situation, the control circuit 110 uses a constant peak currentalgorithm to generate signals different from the PWM signals to controlthe operation of the first switch Q1 and the second switch Q2.

Specifically, in the FIG. 1 example, the control circuit 110 includes adetector 130, a controller 140 and a bias adjustment module 150 coupledtogether as shown in FIG. 1.

The detector 130 is configured to detect various parameters in theelectronic system 100, such as the voltage level of the rectifiedvoltage V_(RECT), the drain voltage of the second switch Q2 (V2) and thecurrent flowing through the first switch Q1 (I_(Q)). In the FIG. 1example, the electronic system 100 includes a voltage divider 104. Thevoltage divider 104 includes two resistors R1 and R2 coupled in seriesto provide a fraction of the rectified voltage V_(RECT) to the detector130 to detect the voltage level of the rectified voltage V_(RECT). In anembodiment, based on the voltage level of the rectified voltageV_(RECT), the detector 130 detects a dimming characteristic, for examplea dimming angle, of the dimmer 102. When the dimming angle is largerthan a threshold angle, the dimmer 102 is in the deep dimming situationin an example.

Further, in the FIG. 1 example, the current sensing resistor R4 has arelatively small resistance such that a voltage drop (V3) on the currentsensing resistor R4 is very small compared to the rectified voltageV_(RECT). The voltage drop V3 is indicative of the current I_(Q), and isprovided to the detector 130 to detect the current I_(Q).

The controller 140 receives the dimming characteristic, the currentI_(Q) and the voltage V2, and controls operations of the first switch Q1and the second switch Q2 based on the received information. In the FIG.1 example, the controller 140 determines operation modes for the firstswitch Q1 and the second switch Q2. For example, at a time duringoperation, the controller 140 determines one operation mode out of anoff mode, a linear mode and a saturation mode for the first switch Q1and determines one operation mode out of an off mode and an on mode forthe second switch Q2. Based on the operation modes, control voltages tothe first switch Q1 and the second switch Q2 are generated. In the FIG.1 example, the bias adjustment module 150 generates and provides thegate voltage for the first switch Q1 based on the operation mode for thefirst switch Q1, and a buffer B generates and provides the gate voltagefor the second switch Q2 based on the operation mode for the secondswitch Q2.

In the FIG. 1 example, the bias adjustment module 150 receives areference bias voltage having a relatively high voltage level, and theoperation mode for the first switch Q1, and adjusts the bias voltage tothe gate of the first switch Q1 based on the operation mode. In anexample, the reference bias voltage is generated based on the rectifiedvoltage V_(RECT) by a bias circuit 105. The bias circuit 105 includes aresistor R3, a diode D1 and a capacitor C1 coupled together as shown inFIG. 1. The capacitor C1 is charged by the rectified voltage V_(RECT),and maintains a relatively stable voltage level, such as about the peaklevel of the rectified voltage V_(RECT).

In an embodiment, when the operation mode for the first switch Q1 is theoff mode, the bias adjustment module 150 outputs a first voltage of arelatively low voltage level, such as about the ground level, as thebias voltage to the gate terminal of the first switch Q1. When theoperation mode for the first switch Q1 is the linear mode, the biasadjustment module 150 outputs a second voltage of a relatively highvoltage level, such as the reference bias voltage, as the bias voltageto the gate terminal of the first switch Q1 in an example. When theoperation mode for the first switch Q1 is the saturation mode, the biasadjustment module 150 outputs a third voltage at an intermediate levelbetween the reference bias voltage and the ground. In an example, thedetector 130 detects a source terminal voltage level of the first switchQ1 (V2), and the voltage at the intermediate level is determined basedon V2, the threshold voltage of the first switch Q1 and the currentI_(Q) flowing through the first switch Q1.

When the operation mode for the second switch Q2 is the off mode, thebuffer B provides a suitable low voltage level to the gate terminal ofthe second switch Q2 to switch off the second switch Q2, such that thesecond switch Q2 does not conduct current; and when the determinedoperation mode for the second switch Q2 is the on mode, the buffer Bprovides a suitable high voltage level to the gate of the second switchQ2 to turn on the second switch Q2, such that the second switch Q2conducts current.

In an embodiment, the controller 140 is implemented as softwareinstructions executed by a processor. In another embodiment, thecontroller 140 is implemented by hardware.

According to an aspect of the disclosure, during operation when thedimmer 102 is in the deep dimming situation, the control circuit 110uses a constant peak current algorithm to generate signals differentfrom traditional PWM signals to control the operation of the firstswitch Q1. Traditional PWM signals transit between a high voltage leveland a low voltage level with pulse width modulated. In the FIG. 1example, the control signal to the gate terminal of the first switch Q1has an intermediate voltage level.

Specifically, in an example, control signals are provided to the gateterminals of the first switch Q1 and the second switch Q2 at a highswitching frequency, such as 200 KHz in an example. In each switchingcycle, in an example, the control circuit 110 first provides the secondvoltage to the gate terminal of the first switch Q1 and also turns onthe second switch Q2. The current I_(Q) starts to increase, and electricenergy is stored in the inductor L. The control circuit 110 monitors thecurrent I_(Q), when the current I_(Q) reaches a predetermined limit,such as when V3 is about 0.4V in an example, the control circuit HOprovides the third voltage to the gate terminal of the first switch Q1and keeps turning on the second switch Q2. The first switch Q1 is in thesaturation mode to consume electric energy, and convert the electricenergy to thermal energy. In an example, a time duration for thesaturation mode is predetermined for the first switch Q1 to consumeenough electric energy in order to sustain the holding current of thedimmer 102 in the deep dimming situation. After the predetermined timeduration for the saturation mode, the control circuit 110 provides thefirst voltage to the gate terminal of the first switch Q1 to turn offthe first switch Q1, and also turns off the second switch Q2. The storedelectric energy in the inductor L is then transferred to the load 109and the capacitor C2.

FIG. 2 shows a flowchart outlining a process example 200 according to anembodiment of the disclosure. In an example, the process 200 is executedin the electronic system 100, such as by the control circuit 110, andthe like. The process starts at S201 and proceeds to S210.

At S210, a dimming characteristic is detected and the process proceedsdifferently based on the dimming characteristic. In the FIG. 1 example,the detector 130 detects a dimming angle. When the dimming angle islarger than a deep dimming threshold, the dimmer 102 is in the deepdimming situation and the electronic system 100 requires power bleedingin order to ensure proper operation of the dimmer 102, and the processproceeds to S220, otherwise, the process proceeds to S250.

At S220, a switch is biased in a linear mode to store electric energy ina magnetic component. In the FIG. 1 example, in a switching cycle, thecontrol circuit 110 first provides the second voltage to the gateterminal of the first switch Q1, thus the first switch Q1 is in thelinear mode. In addition, the control circuit 110 turns on the secondswitch Q2. The current I_(Q) starts to increase, and electric energy isstored in the inductor L.

At S230, the switch is biased in a saturation mode to bleed power by theswitch. In the FIG. 1 example, the control circuit 110 monitors thecurrent I_(Q), when the current I_(Q) reaches a predetermined limit,such as when V3 is about 0.4V in an example, the control circuit 110provides the third voltage to the gate terminal of the first switch Q1,thus the first switch Q1 is in the saturation mode. In addition, thecontrol circuit 110 keeps turning on the second switch Q2. The currentI_(Q) keeps flowing through the first switch Q1, and the first switch Q1has a relatively large source-drain voltage. The first switch Q1consumes electric energy, and converts the electric energy to thermalenergy.

At S240, the switch is bias in the off mode to release the stored energyin the magnetic component to the load. In the FIG. 1 example, after apredetermined time duration for the saturation mode, the control circuit110 provides the first voltage to the gate terminal of the first switchQ1 to turn off the first switch Q1, and also turns off the second switchQ2. The stored electric energy in the inductor L is then transferred tothe load 109 and the capacitor C2. In an example, the S220-S240 arerepetitively executed in each switching cycle. When the dimmingcharacteristic changes, the process returns to S210.

At S250, the switch is biased in the linear mode to store electricenergy in a magnetic component. In the FIG. 1 example, in a switchingcycle, the control circuit 110 first provides the second voltage to thegate terminal of the first switch Q1, thus the first switch Q1 is in thelinear mode. In addition, the control circuit 110 turns on the secondswitch Q2. The current I_(Q) starts to increase, and electric energy isstored in the inductor L.

At S260, the switch is bias in the off mode to release the stored energyin the magnetic component to the load. in the FIG. 1 example, thecontrol circuit 110 monitors the current I_(Q), when the current I_(Q)reaches a predetermined limit, such as when V3 is about 0.4V in anexample, the control circuit 110 provides the first voltage to the gateterminal of the first switch Q1 to turn off the first switch Q1. Thecontrol circuit 110 also turns off the second switch Q2. Then, thestored electric energy in the inductor L is transferred to the load 109and the capacitor C2. In an example, the S250-S260 are repetitivelyexecuted in each switching cycle. When the dimming characteristicchanges, the process returns to S210.

FIG. 3 shows a plot 300 of waveforms for voltages and current in theelectronic system 100 according to an embodiment of the disclosure. Theplot 300 includes a first waveform 310 for the gate voltage of thesecond switch Q2 (V1), a second waveform 320 for the source voltage ofthe first switch Q1 (V2), a third waveform 330 for the gate-sourcevoltage of the first switch Q1 (V4), a fourth waveform 340 for thesource-drain voltage of the first switch Q1 (V5), and a fifth waveform350 for the current I_(Q).

When the dimmer 102 is not in a deep dimming situation, the voltages andcurrent in the electronic system 100 are illustrated by a first portionof the waveforms 310-350 in a first switching cycle (SWITCHING CYCLE 1).In an example, the first portion of the waveforms 310-350 repeats foreach switching cycle when the dimmer 102 is not in the deep dimmingsituation. When the dimmer 102 is in a deep dimming situation, thevoltages and current in the electronic system 100 are illustrated by asecond portion of the waveforms 310-350 in a second switching cycle(SWITCHING CYCLE 2). In an example, the second portion of the waveforms310-350 repeats for each switching cycle when the dimmer 102 is in thedeep dimming situation.

Specifically, when the dimmer 102 is not in the deep dimming situation,in each switching cycle, the control circuit 110 first provides arelatively high gate voltage to the second switch Q2 as shown by 311,the second switch Q2 is turned on, and the source voltage of the firstswitch Q1 is low as shown by 321. In addition, the control circuit 110provides a relatively high gate voltage to the first switch Q1 as shownby 331, thus the first switch Q1 is in the linear mode with a relativelysmall source-drain voltage (e.g., about zero) as shown by 341 andconsumes little power. The current I_(Q) starts to increase as shown by351, and electric energy is stored in the inductor L.

Further, the control circuit 110 monitors the current I_(Q), when thecurrent I_(Q) reaches a predetermined limit, such as when V3 is about0.4V in an example, the control circuit 110 provides a relatively lowgate voltage for the second switch Q2 as shown by 312, the second switchQ2 is turned off, and the source voltage of the first switch Q1 is highas shown by 322. In addition, the control circuit 110 provides arelatively low gate voltage to the first switch Q1 as shown by 332, thusthe first switch Q1 is in the off mode to shut off the current I_(Q) asshown by 352, and consumes little power. The stored electric energy inthe inductor L is then transferred to the load 109 and the capacitor C2.

When the dimmer 102 is in the deep dimming situation, in each switchingcycle, the control circuit 110 first provides a relatively high gatevoltage to the second switch Q2 as shown by 313, the second switch Q2 isturned on, and the source voltage of the first switch Q1 is low as shownby 323. In addition, the control circuit 110 provides a relatively highgate voltage to the first switch Q1 as shown by 333, thus the firstswitch Q1 is in the linear mode with a relatively small source-drainvoltage (e.g., about zero) as shown by 343 and consumes little power.The current I_(Q) starts to increase as shown by 353, and electricenergy is stored in the inductor L.

Further, the control circuit 110 monitors the current I_(Q), when thecurrent I_(Q) reaches a predetermined limit, such as when V3 is about0.4V in an example, the control circuit 110 maintains the relativelyhigh gate voltage for the second switch Q2 as shown by 314, the secondswitch Q2 is still turned on, and the source voltage of the first switchQ1 is low as shown by 324. In addition, the control circuit 110 providesan intermediate gate voltage to the first switch Q1 as shown by 334,thus the first switch Q1 is in the saturation mode to conduct thecurrent I_(Q) as shown by 354 with a relatively large source-drainvoltage as shown by 344. Thus, the first switch Q1 consumes power andconverts electric energy to thermal energy.

Further, after a time duration, the control circuit 110 provides arelatively low gate voltage for the second switch Q2 as shown by 315,the second switch Q2 is turned off, and the source voltage of the firstswitch Q1 is high as shown by 325. In addition, the control circuit 110provides a relatively low gate voltage to the first switch Q1 as shownby 335, thus the first switch Q1 is in the off mode to shut off thecurrent I_(Q) as shown by 355, and consumes little power. The storedelectric energy in the inductor L is then transferred to the load 109and the capacitor C2.

While aspects of the present disclosure have been described inconjunction with the specific embodiments thereof that are proposed asexamples, alternatives, modifications, and variations to the examplesmay be made. Accordingly, embodiments as set forth herein are intendedto be illustrative and not limiting. There are changes that may be madewithout departing from the scope of the claims set forth below.

1. An apparatus, comprising: an energy transfer circuitry configured totransfer energy from an energy source to a load; control circuitryconfigured to detect a dimming characteristic in the energy source; afirst switch configured to be biased by the control circuitry to consumean energy from the energy source based on the dimming characteristic;and a second switch coupled with the first switch in series togetherforming a current path to bleed the enemy.
 2. The apparatus of claim 1,wherein the control circuitry is further configured to draw additionalenergy from the energy source to the load for sustaining the dimmingcharacteristic.
 3. The apparatus of claim 2, wherein the controlcircuitry is further configured to switch on/off the first switch todraw the additional energy from the energy source to the load.
 4. Theapparatus of claim 3, wherein both the first and second switches arecoupled in series with the energy transfer circuitry.
 5. The apparatusof claim 4, wherein the first switch is a metal oxide semiconductorfield effect transistor (MOSFET), and wherein the cot circuitry isfurther configured to bias the MOSFET in a saturation mode to consumethe energy on the MOSFET.
 6. The apparatus of claim 5, wherein thecontrol circuitry is further configured to bias the MOSFET in a linearmode to turn on the MOSFET to store the additional energy in the energytransfer circuitry in connection with the MOSFET.
 7. The apparatus ofclaim 6, wherein the control circuitry is further configured to bias theMOSFET in an off mode to turn off the MOSFET to transfer the additionalenergy from the energy transfer circuitry to the load.
 8. The apparatusof claim 7, wherein the control circuitry is further configured tooutput a first voltage for a gate terminal of the MOSFET to bias theMOSFET in the linear mode and to detect a current flowing through theMOSFET.
 9. The apparatus of claim 8, wherein the control circuitry isfurther configured to output a second voltage for the gate terminal ofthe MOSFET to bias the MOSFET in the saturation mode in order to consumethe energy when the current reaches a limit and to output a thirdvoltage for the gate terminal of the MOSFET to turn off the MOSFET. 10.The apparatus of claim 2, wherein the control circuitry is furtherconfigured to switch on the first switch and the second switch to storethe energy in the energy transfer circuitry and to switch on the firstswitch and switch off the second switch to charge a capacitor thatstores energy for driving an integrated circuit (IC) chip including thecontrol circuitry.
 11. A method, comprising: transferring energy from anenergy source to a load by an energy transfer circuitry; detecting adimming characteristic in the energy source by a control circuitry;biasing a first switch by the control circuitry to consume an energyfrom the energy source based on the dimming characteristic; and bleedingthe energy via a current path formed of a second switch coupled togetherwith the first switch in series.
 12. The method of claim 11, furthercomprising: drawing additional energy from the energy source to the loadfor sustaining the dimming characteristic.
 13. The method of claim 12,further comprising: switching on/off the first switch to draw theadditional energy from the energy source to the load.
 14. The method ofclaim 13, further comprising: coupling both the first and secondswitches in series with the energy transfer circuitry.
 15. The method ofclaim 14, further comprising: biasing the first switch, which is a metaloxide semiconductor field effect transistor (MOSFET), in a saturationmode to consume the energy on the MOSFET.
 16. The method of claim 15,further comprising: biasing the MOSFET in a linear mode to turn on theMOSFET to store the additional energy in the energy transfer circuitryin connection with the MOSFET.
 17. The method of claim 16, furthercomprising: biasing the MOSFET in an off mode to turn off the MOSFET totransfer the additional energy from the energy transfer circuitry to theload.
 18. The method of claim 17, further comprising: generating a firstvoltage for a gate tem of the MOSFET to bias the MOSFET in the linearmode; and detecting a current flowing through the MOSFET.
 19. The methodof claim 18, further comprising: generating a second voltage for thegate terminal of the MOSFET to bias the MOSFET in the saturation mode inorder consume the energy when the current reaches a limit and to outputa third voltage for the gate terminal of the MOSFET to turn off theMOSFET.
 20. The method of claim 12, further comprising: switching on thefirst switch and the second switch to store the energy in the energytransfer circuitry; and switching on the first switch and switching offthe second switch to charge a capacitor that stores energy for drivingan integrated circuit (IC) chip including the control circuitry.